A combination of a highly non-linear transfer function, 4:2:0 subsampling and non-constant luminance ordering gives rise to severe artifacts in saturated colors in pictures of a video sequence. A non-linear transfer function converts linear samples to non-linear samples with the purpose to mimic human vision.
A simple example of a non-linear transfer function is x(1/gamma), where gamma is 2.2. An example of another transfer function is the one used in Society of Motion Picture & Television Engineers (SMPTE) specification ST 2084 [1]. Before display the inverse of the non-linear transfer function is typically used, but it is also possible to use a function that is not the inverse of the non-linear transfer function. In the gamma example xgamma can be used to go back to linear samples.
One example is the way to carry out conversion from RGB 4:4:4 to Y′CbCr 4:2:0 that is described in [2], which we will refer to as the “anchor” way of processing in this document. RGB 4:4:4 is the color format typically used by cameras to capture video and by displays to present the video. To compress the video with less perceptual artifacts, RGB 4:4:4 is typically converted to Y′CbCr 4:2:0 before compression. In this case, RGB 4:4:4 is transferred by a non-linear transfer function to R′G′B′ 4:4:4 which then is converted to Y′CbCr 4:4:4 by a linear color transform. Finally, the chroma samples Cb and Cr are subsampled, by a factor two in both vertical and horizontal directions, to quarter resolution resulting in Y′CbCr 4:2:0. As described in Annex B, the anchor way of processing gives rise to situations where changes between two colors of similar luminance can result in a reconstructed picture or image with very different luminances.